Experimental Study of Flow Around a Rotating Sphere at a Moderate Reynolds Number

Aerodynamic forces on a rotating golf ball were measured in a water tunnel for a Reynolds number of Re_D = 8000 at a spinning ratio V_s/U_o (the surface velocity over the freestream velocity) between 0 and 6.0. The flow fields downstream of a spinning sphere were also measured using a time-resolved stereoscopic PIV system with field-of-views perpendicular to the incoming flow to capture the three velocity components.There appeared to be several critical spinning ratios at V_s/U_o ≈ 0.75, 2.0 and 3.0. Both lift and drag increased quickly for V_s/U_o ≲ 0.75; there was a sudden drop in lift at V_s/U_o ≈ 0.75; lift increased quickly while drag increased slowly for 1.0 ≲ V_s/U_o ≲ 2.0; both lift and drag plateaued for 2.0 ≲ V_s/U_o ≲ 3.0; lift increased while drag decreased for large spinning ratios 3.0 ≲ V_s/U_o ≲ 6.0. Flow measurements suggested the lift increase was associated with a downwash downstream of spinning ball. Down wash induced a pair of counter-rotating vortices that caused increases in drag, similar to the induced drag on a finite-span wing. Boundary layer transition occurred on retreating side at a large spinning ratio 3.0 ≲ V_s/U_o ≲ 6.0, the downward appeared to be weaker in this situation and drag increased thus became smaller again.